A conventional prior art hydraulic damper or shock absorber comprises a cylinder defining a fluid chamber having a piston slidably disposed in the fluid chamber with the piston separating the interior of the cylinder into an upper and a lower working chamber. A piston rod is connected to the piston and extends out of one end of the cylinder. A first valving system is incorporated for generating damping force during the extension or rebound stroke of the hydraulic damper and a second valving system is incorporated for generating damping force during the compression stroke of the hydraulic damper.
Various types of damping force generating devices have been developed to generate desired damping forces in relation to the frequency of the inputs from the roads over which the vehicle travels. These frequency dependent selective damping devices provide the ability to have softer damping characteristics with higher frequency road inputs. These softer damping characteristics lead to a more effective isolation of the vehicle body from unwanted disturbances. Typically these frequency dependent damping devices operate only during an extension or a rebound movement of the hydraulic damper or shock absorber and some operate during both an extension and a rebound movement of the hydraulic damper or shock absorber.
The continued development of hydraulic dampers includes the development of frequency dependent damping devices that improve the function in an extension/rebound movement or a compression movement of the hydraulic damper or shock absorber.